Centrifugal pump throttling controller



Feb. 23, 1960 G. P. JENNINGS CENTRIFUGAL PUMP THROTTLINGY CONTROLLER 2 Sheets-Sheet 1 Filed Jan. 13, 1958 PRESSURE DIFFERENTIAL THROTTLING 360 PSI PRESSURE DIFFERENTIAL LINE PRESSURE REQUIREI7/ IO 20 3O 4O 50 PUMPING RATE IN I000 BPD IIOO FIG.

INVENTOR. G.P. JENNINGS BY W ,4 T TORNE rs v Feb. 23, 1960 G. P. JENNINGS 2,925,784

I CENTRIFUGAL PUMP THROTTLING CONTROLLER Filed Jan. 15, 1958 2 Sheets-Sheet 2 I REVERSING -Q5 CONTACTOR I I I IUD x4 INVENTOR.

G.P. JENNINGS BYMu A T TOR/VEYS CENTRIFUGAL PUMP THROTILING I (IONTROLLER Gerald P. Jennings, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Application January 13, 1958, Serial No. 708,536 Claims. (Cl. 103-97 This invention relates to pipe line operation. In one aspect it relates to operation of multistage centrifugal pipe line pumps driven by constant speed prime movers for pumping liquids at Various rates. In another aspect it relates to operation of such pumps wherein one or more stages are removed from service when pumping liquids at reduced rates in order to efiect a saving in horsepower. In still another aspect it relates to means of reducing the horsepower loss when pumping liquid at reduced rates with multistage centrifugal pumps operating at constant speed.

In pipe line work it is quite common to use constant speed electric motors as prime movers to drive centrifugal pumps. Since such pumps are required to put up relatively high pressures multistage pumps are ordinarily used. Pumps are ordinarily installed which will transfer liquids at the highest expected rate at'a predetermined speed. For economic reasons such pumps are frequently powered by constant speed electric motors. When operating at full rate, no pressure control is required. However, when operating such multistage centrifugal pumps at the constant speeds of the constant speed prime movers at reduced pumping rates, or on a petroleum product pipe line, for example, with products of varying specific gravity and viscosity, it is often necessary to throttle the discharge of such pumps by throttling a valve in the discharge line from the pump. The throttling of discharge from such pumps increases pump pressure while actually pumping less than capacity volumes thereby causing a loss of power.

This invention provides means for reducing the output of such centrifugal pumps without experiencing the usual large losses in power. According to this invention power losses are reduced to the extent of about 60 to 65 percent. When saving 60 to 65 percent of the power required to operate one or more stages of a high pressure centrifugal pump driven by a prime mover of, for example, 1400 horsepower, such a saving is appreciable.

An object of this invention is to provide apparatus for reducing the horsepower loss when pumping liquid with constant speed centrifugal pumps at rates lower than maximum designed pump rates.

In another aspect this invention relates to apparatus for carrying out the aforementioned object.

Another object of this invention is to provide apparatus for automatically blocking one or more stages of a multistage centrifugal pump from use when pumping liquid at less than capacity rates.

Other objects and advantages of this invention will be realized upon reading the following description with the attached drawing which, respectively, describes and illustrates a preferredembodimeut of the invention.

In the drawing Figure 1 is a chart illustrating pump performance curves, horsepower requirement curves and the like.

Figure 2 illustrates in diagrammatic form one embodiment of this invention.

Figure 3 illustrates another embodiment of this in- Vention.

2,925,784 Patented Feb. 23, 1960 Figure 4 illustrates, in detailed form, a portion of the apparatus of Figures 2 and 3.

This invention is directed specifically to apparatus for reducing the power required by a multistage centrifugal pump operated at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at said constant speed comprising, in combination, a multistage centrifugal pump, a constant speed prime mover, said prime mover being adapted to drive said pump at a constant speed, an inlet in the first stage of said pump for liquid to be pumped, a first outlet in the last stage of said pump for outlet of pumped liquid, 21 second outlet in an intermediate stage for outlet of pumped liquid, a check valve communicating with said second outlet to permit flow of liquid only from said second outlet, a slow operating motor valve communicating with said first outlet, a conduit communicating with said check valve and with said motor valve for passage of pumped liquid, a second valve in said conduit for regulating flow of pumped liquid, and means to actuate said motor valve in response to pressure dilferential across said second valve.

Referring now to the drawing, and specifically to Figure 1, the curve carrying the legend Pump A Pressure Difierential is a curve illustrating the performance of a multistage centrifugal pump designed to pump liquid at a constant rate when driven by a constant speed prime mover. The ordinate, Pump Diflerential in p.s.i. (pounds per square inch), is the pressure differential or pressure of liquid between the outlet and inlet of such a multistage pump. The abscissa is the pumping rate in thousands of barrels per day. The ordinate at the right-hand side of the figure is brake horsepower.

With a multistage centrifugal pump designed to pump 60,000 barrels per day of a given liquid under conventional pipe line high pressure, a prime mover powering the pump and operating at a constant speed may require about 1,400 horsepower. In this case there is no throttling of the pump outlet. When such a liquid is available for pumping at a rate of only 50,000 barrels per day, it is common practice to throttle the discharge of the centrifugal pump to such an extent that the pumping rate is reduced to 50,000 barrels per day. In such a case the discharge of the pump is throttled about 360 p.s.i. As is well understood by those skilled in the art, when throttling the outlet of a centrifugal pump, the pump pressure increases somewhat and the difference in outlet pressure during throttling is reduced by about the aforementioned 360 p.s.i. so that the pipe line pressure downstream of the throttling valve is about 360 p.s.i. less than the pressure upstream of the valve. This throttling Wastes about 395 horsepower. On referring to the curve identified by the legend Horsepower-Pump A, it will be noted that by throttling the discharge of the valve to pass 50,000 barrels per day, the horsepower requirement of the pump is reduced from about 1,400 to about 1,225 horsepower. The curve identified by legend HorsepowerPump B is for a centrifugal pump designed to pump 50,000 barrels per day. Such a pump requires about 830 horsepower, point C on this curve. Point B on the pump A horsepower curve, as mentioned, indicates about 1,225 horsepower is required when throttling a 60,000 barrels per day pump. The difference between 1,225 horsepower and 830 horsepower is 395 horsepower and this is approximately the power lost when using a 60,000 barrels per day pump having its discharge throttled to 50,000 barrels per day rather than using a pump designed for pumping 50,000 barrels per day. This invention is directed to savinga major portion of the aforementioned 395 horsepower by blocking off the high pressure pump stage completely rather than merely throttling the pump discharge. In this manner from 60 to 65 percent of this power loss is saved. This 60 to 65 percent amounts to a saving of about 237 to horsepower. 7 Such an amount of power is well worth savlng.

The point at which the pump A pressure differential curve intersects the Line Pressure Required curve at 60,000 barrels per day gives a pipe line pressure of about 1,050 pounds. When the pump discharge is throttled to 50,000barrels per day, or when a pump designed for transferring 50,000 barrels per day is used, the pipe line pressure is approximately 750 p.s.i.

From another point of view the saving in horsepower may be considered as a saving of at least a portion of the power required to increase the pressure of the liquid being pumped from about 750 to 1,050 psi. However, as mentioned above, when throttling a pump, the pressure upstream of the throttling valve is increased so that the actual power lost in the case illustrated is the difference in power required to put up 1,110 p.s.i. in place of about .750 p.s.i., which is 360 psi. V

In Figure 2 is illustrated an embodiment of my invention whereby apparatus is provided for blocking off the last stage of a multi-stage centrifugal pump inorder to save power when pumping a volume of liquid less than the volume for which the pump was designed. I

Reference numeral 24 identifies a pipelinethrough which liquid is transferred for long distances Reference numeral 11 identifies a 6-stage centrifugal pump,

the several stages being identified by 8-1, 8-2,, S-3, 8-4,

S- and 8-6. Such pumps are frequently designed so that the highest pressure or last stage is as near the middle of the pump as practical. In the case illustrated, a pipe 22 provided with a valve 12 leads liquid to be pumped to the first stage, S4, at one end of the pump. Stage S-2 is next to the first stage while the third stage, 8-3, is at the opposite end of the pump. Stages 5-4,

S5 and S-6v are positioned intermediate the third and second stages. The last or high pressure stage, 8-6, communicates through an outlet 16 to a slow-closing and slow-opening motor valve 18. This valve communicates by way of a pipe 23 to pipe line 24 downstream of a check valve 14 as regards the directionof flow of liquid in the pipe line. Pipe 23 is provided with a valve 13. Valves 12 and 13 are installed in pipes 22 and 23, respectively, in case pump 11 is shut down for servicing or repairs.

The fifth or next to the high pressure stage of the pump is provided with an outlet 17 which communicates with a check valve 19. This check valve 19 passes liquid only from the next to the last stage to pipe 23. An outlet 17' is provided in the fourth stage in case it is ever desired to block oif the two highest pressure stages. Reference numeral 25' identifies a constant speed prime mover for transmitting constant rotational speed through a shaft 25 to the impellers of the pump.

Discharge pipe 23 is provided with a valve between motor valve 18 and valve 13. Conduits or tubes 36 and 37 lead from pipe 23 on opposite sides of valve 15 to a differential pressure controller 26. Pressure air, from a source not shown, is lead to the diiferential pressure controller through a pipe 44. A tube 34 leads from the difierential pressure controller to a pneumatically actuated reversing contactor switch 21. Conduitf42 leads electrical current, from a source not shown, to the reversing contactor switch for actuating the motor of motor valve 18. Conduit 35 is a multistrand cable for conducting electric current from the contactor to the motor of the motor valve.

Since motor valve 18 is intended to close slowly and toopen slowly, it is necessary to provide means for reversingthe current through the motor of the valve and the reversing contactor switch 21 is an apparatus suitable for this purpose.

In Figure 4 is illustrated one form of a reversible contactor switch suitablefor the purpose at hand. In this figure the motor valve 18 is illustrated as being connected to pipes 16 and 23 and cable 35 leading from the motor valve to the reversing contactor. This cable 35 is composed of wires 35' and 35". These wires lead to separate arms of a double-pole, double-throw switch 49. Reference numeral 47 identifies the source of E.M.F. Wires 45, 46 and 45' lead from the source of 47 to the double-pole, double-throw switch, as illustrated. The dilferential pressure controller, as is well understood by those skilled in the art, is an apparatus adapted for regulating air pressure in response to dilferential pressures and is available from instrument supply houses. Thus, in this particular case, the difierential pressure controller 20 is so adjusted that it regulates pressure air in tube 34 so that at a predetermined high pressure differential an expansible bellows 48 expands to approximately its maximum extent, which expansion moves the contact bars of switch 49downward to close the circuit between lead wire 35' andwire 45, and from 'lead wire 35 to lead wire 46 thereby providing flow of electric current in one direction through the motor of the motor valve 18. The diiferential pressure controller 20 is also set so that when it senses a predetermined minimum pressure difierential across valve 15, it reduces the air pressure in tube 34 so that the bellows contracts and raises the switch knives of switch 49 to close the circuit between lead 35 and lead 45 and between lead 35" and lead 45 thereby reversing the flow of current through the motor of valve 18. It is intended, according to the present invention, that upon sensing a predetermined maximum pressure drop through valve 15, the differential pressure controller increases the pressure of the air in tube 34 which lowers the knives of switch 49 to close the circuit so that current flows in such a direction through the motor of the motor valve 18 that the valve closes. It is further intended that upon sensing a predetermined minimum pressure difierential across valve 15, the dilferential pressure controller 20 decreases the pressure of the air in tube 34 thereby permitting the bellows 48 to contract thereby completing the other circuit throughthe motor of valve 18 thereby reversing the flow of electric current and opening the valve.

In the operation of this invention an operator in the vicinity of pump 11 or at some remote point, at a time when he is aware that pump 11. is to pump a reduced volume of liquid, throttles valve 15 to cause a pressure drop through this valve slightly greater than the pres sure differential developedby the last stage, S6, of the pump. For example, if the last stage of the pump develops approximately 250.p.s.i., then the valve 15 is throttled to such an extent that the pressure drop through the valve is approximately 275 psi. The differential pressure controller 20 is adjusted so that when it senses a pressure drop across valve 15 of about 275 psi, pressure air is increased in tube 34 to expand bellows 48 to lower the knives of switch 49 .to close the lower contacts of the switch to close valve 18.

During the closing of motor valve 18 the pressure on the downstream side of this motor valvegradually decreasesand at the same time the diiferential across valve 15 willdecrease. When the pressure on-the downstream side'of motor valve 18 drops to the discharge pressure of the next to the lastpump stage, S5, the check valve 19 will open and then when the motor operated valve 18 closes completely, the last pump stage is cut out. With the valve 15 remaining at its last setting, this valve is now throttling the discharge only slightly because the volume of liquid passing through the valve, for example, was reduced to 50,000 barrels per day from 60,000 barrels per day. Operation remains constant at the reduced rate until such time that it is, desired to increase the volume of liquid passing through pump 11.

When it is desired to increase the pump rate, it is merely necessary to open slowly valve to reduce the pressure differential across this valve to the aforementioned predetermined low pressure diflerential. Controller reduces the air pressure in tube 34 which, in turn, permits bellows 48 to contract thereby raising the knives of switch 49 to close the circuit which allows flow of electric current in the reverse direction thereby causing the motor valve 18 to open slowly. During the time the motor valve is opening the pressure downstream of this valve increases andas soon as this increase in pressure is evident in pipe 23 the check valve 19 closes thereby preventing outlet of liquid from the fifth, stage of the pump. When valve is 18 is fully opened, and because valve 15 is substantially fully opened, the pressure drop through this latter valve is between the aforementioned predetermined minimum and maximum values. Under these conditions the prime mover is actuating pump 11 with the delivery of its maximum volume of liquid.

Again referring to the reversing contactor 21, which is illustrated in detail in Figure 4, it is intended that when the knife blades of switch 49 are lowered to contact lead wires 46 and 45', motor valve 18 becomes closed. This motor valve remains closed even though the knife blades of switch 49 are raised by contraction and remains opened irrespective of the position of the knives of switch 49 as long as they are not lowered to contact wires 46 and 45'.

It is preferred that a tension spring be provided within bellows 48 to make certain that the bellows contract properly to raise the switch upon reduction of pressure within the bellows. Means need not be provided to expand the bellows other than provision of proper air pressure within the bellows.

If desired, two or more stages of the centrifugal pump can be blocked out of operation by merely providing outlet 17' with a check valve 19 and disconnecting or removing check valve 19 from the stages-5 of the pump. In other words, pipe 23 is attached by way of a check valve similar to valve 19 to outlet 17 in place of to outlet 17. In this manner two stages of the pump are blocked off in place of one stage, as hereinabove described. In this latter case the maximum pressure differential which operates through differential pressure controller 20 to lower the knife blades of switch 49 is slightly greater than the combined pressure put up by stages S-5 and S6. Otherwise the operation is similar to that hereinabove described. In Figure 3 is illustrated apparatus which is, in general, similar to that described relative to Figure 2 but which provides apparatus for closing 011 under a somewhat reduced pump throughput the high pressure stage of the pump and, upon further reduction of pump discharge volume, the next to the last pump stage is blocked otf while the check valve 27 is attached to the outlet of the fourth stage of the pump. The fourth pump stage then is the final pump stage. In this case motor valve 18 is inserted between outlet 16 and pipe 23 similar to that illustrated in Figure 2. Another slow-opening and slow-closing motor valve 26 is inserted between outlet 17 of the fifth stage and pipe 23 while the check valve 27, as mentioned, is inserted between outlet 17' and pipe 23. In this case a pair of difierential pressure controllers 28 and 30 are provided along with a pair of reversing contactors 29 and 31. These several pieces of equipment are installed as illustrated. Tubes 38 and 40 conduct pressures from downstream and upstream of valve 15 to the differential pressure controller 28. Tubes 39 and 41 conduct pressures from downstream and upstream, respectively, of valve 15 to the differential pressure controller 30. Leads 32 and a 6 33 conduct electric current, from a source not shown, to the reversing contactors. These reversing contactors and differential pressure controllers of Figure 3 operate, in general, similar. to the analogous parts of Figure 2. The first pressure dilferential controller 28 and its reversing contactor 29 operate in a manner similar to that described relative to Figure 2. The upper and lower limits of pressure diiferentials which actuate thesecond differential pressure controller 30 are slightly higher than the corresponding pressure differentials which actuate the differential pressure controller 28 When in operation the pressure differential controller 28 operates first to block out the last centrifugal pump stage and when valve 15 is further throttled to limit pump output, the still greater pressure difierential across this valve actuates the differential pressure controller 30 which then actuates reversing contactor 31 to block out the next highest pump stage, S 5, and this stage 8-5 is then blocked fiom operation and the pump stage 8-4 then becomes the outlet of the pump.

When the valve 15 is opened so as to decrease the pressure differential across the valve to a minimum somewhat higher than the minimum which actuates the pressure differential controller 28, the controller 30 actuates reversing contactor 31 to open valve 26. Then, if valve 15 is further opened, the pressure difierential across the valve further decreases to the minimum at which controller 28 is set to actuate and, when this differential is reached, electric current is reversed by contactor 29 and valve 18 is opened thereby putting the final pump stage into operation.

This 2-step method is not necessarily confined to a single multi-stage pump but, in cases where there are two or more pumps in a pump station, the second step can be applied to the last pump stage of a second pump. In fact, the above-explained multistep method of removing pump stages can be applied to multipump operation for closing down the highest stage pump first, followed by the next stage pump, etc.

Valve 15 has been herein referred to as merely being opened or closed without any reference whatever as to whether the valve is a manually operable valve or a motor valve. If desired, valve 15 is a manually operable valve; and with an operator at hand, when it is desired to reduce the pump output, the operator merely'closes valve 15 by manual openation to such an extent that the diflerential pressure controller 20 of Figure 2 operates to block out the last stage of the pump. Likewise, when it is desired to increase the pump transfer, the operator merely opens the valve manually which operation then causes motor valve 18 to open with the result that the last stage 8-6 is placed in operation.

If desired, valve 15 can be a motor valve which is actuated by push buttons or other means in the general vicinity of pump 11; or the push buttons 50 can be installed at some remote point. Leads 43 conduct a signal from the push buttons to actuating apparatus for the valve. As an illustration of a remote point for installation of the push buttons for actuation of valve 15 when it is a motor valve, the push buttons can 'be installed at the first pump station, for example, along a pipe line, and by pushing one button or the other it is possible to open or to close motor valve 15 at a second or third or other pump station down the pipe line. There is substantially no limit to the distances between motor valve 15 and the push buttons. In this manner one operator at a central pump station or at the first or other pump station along the pipe line can actually operate all the pumps along the length of the pipe line.

It is realized that such a pump control system as herein disclosed has a considerably wider application than herein described. It has an application under circumstances in which volume flow of liquid from a constant speed centrifugal pump is reduced from a maximum to some minimum value with the result that saving of power is obtained. In one application of the present invention, when crude oil is pumped in a pipe line at, for example, the above-mentioned 60,000 barrels per day, all stages of the pump are in usewhen pumping at full capacity. If for some reason crude oilis available at a pumping rate of only 50,000 barrels per day, then the present invention has utility for saving of horsepower in pumpingthe smaller volume of the liquid; The above-mentioned pumping rates of 60,000 and 50,000 barrels per day are used merely as an illustration of the. utility of this invention because designed pump throughputs of more or less than 60,000 barrels are many times employed.

The-invention also has utility in a petroleum product pipe line. In this case, for example, if kerosene, which has a relatively high specific gravity and a' high viscosity. is pumped at, forexample, 60,000 barrels per day, all stages of the pump may be used. When it is desired to make a pumping of, for example, gasoline following the kerosene, adjustment of the pump is desirable. Since gasoline has a lower specific gravity than kerosene and also a lower viscosity, considerably less pump pressure is required to move even an equal volume of gasoline through the pipe line. In this case the pressure drop of the lower specific gravity and lower viscosity liquid through valve 15 is less than the pressure drop through this valve caused by the same volume of kerosene and under this reduced pressure diflferenti al of gasoline, the pressure controller, such ascontroller 20 of Figure 2, is set to close motor valve 18 and block out the final stage of the pump because, for example, five stages of the pump can, in many instances, pump a volume of gasoline equal to the volume of kerosene pumped by six stages.

Motor valves 15, 18 and 26 are herein disclosed as being electrically operated valves. Other types ofmotor valves, such as pneumatically operated and hydraulically operated valves, are, under some conditions, used in place of the electrically operated valves. Detailed descriptions of other types of valve control systems for operating the electrically operated valves 15,18 and 26, and for operating pneumatically and hydraulically operated valves, are believed not necessary because the need, installation and care of such systems are well understood by those skilled in the control instrument art.

As will be evident to those skilled in the art, various modifications of this invention can be made, or followed,

connected to said pump to drive said pump at a constant speed, an inlet in the first stage of said pump for liquid to be pumped, a firstoutlet in the last stage of said pump for out-let of pumped liquid, a second outlet in an intermediate stage for outlet of pumped liquid, a check valve in said second outlet to permit flow of liquid only from said second outlet, a slow operating motor valve in said first outlet, a conduit connecting the outlet side of said check valve and the outlet side of said motor valve for passage of pumped liquid, a second valve in said conduit on the downstream side of said motor valve and of said in the light of this foregoing disclosure andjdiscussion,

without departing from the spirit or scope thereof.

I claim:

1. Apparatus for reducing the power required by a multistage centrifugal pump operated at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at said constant speed comprising, in combination, a multistage centrifugal pump, a constant speed prime mover, said prime mover being connected to said pump to drive said pump at a constant speed, an inlet in the first stage of, said pump for liquid to be pumped, a first outlet in the last stage of said pump for outlet of pumped liquid, a second outlet in an intermediate stage for outlet of pumped liquid, a check valve in said second outlet to permit flow of liquid only from said second outlet, a slow operatingmotor valve in said first outlet, a conduit connecting the outlet side of said check valve and the outlet side of said motor valve for passage of pumped liquid, a second valve in said conduit on the downstream side of said motor valve and of said check valve as regards direction of flow of pumped liquid, and means to actuate said motor valve in response to pressure differential across said second valve.

2. Apparatus for reducing the power required by a multistage centrifugal pump operated. at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at. said constant speed comprising, incombination, aimultistage centrifugal pump, a constant speed prime mover, said prime mover being check valve as regards direction of flow of pumped liquid,

and a differential pressure controller, said controller being operably connected with said conduit on either side of said second valve and. with said motor valve, and said controller being adapted to actuate said motor valve in response to pressure differential across said second valve.

3. Apparatus for reducing. the power required by a multistage centrifugal pump operated at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at said constant speed comprising, in combination, a multistage centrifugal pump, a constant speed prime mover, said prime mover being connected to said pump to drive said pump at a constant speed, an inlet in the first stage of said pump for liquid to be pumped, a first outlet in the last stage of said pump for outlet of pumped liquid, a second outlet in an intermediate stage for outlet of pumped liquid, a check valve in said second outlet to permit flow of liquidonly from said second outlet, at slow operating motor valve in said first outlet, a first conduit connecting the outlet side of said check valve and with the outlet side of said motor valve, 21 second conduit connected with said first conduit for passage, of pumped liquid, a second valve in said second conduit for regulating flow of pumped liquid, and a diiferential pressure controller, a reversing contactor operably connected with said controller, separate conduits connecting said controller with said second conduit on either side of said second valve, whereby said controller actuates said reversing contactor to open and to close said motor valve in response to pressure drop across said second valve.

4. In the apparatus of claim 1 wherein said second valve is a manually operable valve.

5. In the apparatus of claim 1 wherein said second valve is a remotely operable motor valve.

6. In a pipe line pumping system, an apparatus for reducing the power required by a multistage centrifugal pump operated at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at said constant speed, comprising, in combination, a pipe line, said pipe line being adapted for long distance transmission of liquids, a first valve in said pipe line, a multistage centrifugal pump, a constant speed prime mover, said prime mover being connected to said pump to rotate the impellers of said pump at a constant speed, an inlet in the first stage of said pump for liquid to be pumped, a first outlet in the last stage of said pump for outlet of pumped liquid, a second outlet in an intermediate stage for outlet of pumped liquid, a check valve in said second outlet to permit flow of liquid only from said second outlet, a slow operating motor valve in said first outlet, a first conduit communicating said pipeline on the upstream side of said first valve as regards direction of flow of liquid in said pipe line withsaid inlet, a second conduit communicating said motorvalve with said pipe line on the downstream side of said first valve, a third conduit communicating said check valve with said second 9 ducing the power required by a multistage centrifugal pump operated at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at said constant speed, comprising, in combination, a pipe line, said pipe line being adapted for long distance transmission of liquids, a first valve in said pipe line, a multistage centrifugal pump, a constant speed prime mover, said prime mover being connected to said pump to rotate the impellers of said pump at a constant speed, an inlet in the first stage of said pump for liquid to be pumped, a first outlet in the last stage of said pump for outlet of pumped liquid, a second outlet in an intermediate stage for outlet of pumped liquid, a check valve in said second outlet to permit flow of liquid only from said second outlet, 2. slow operating motor valve in said first outlet, a first conduit communicating said pipe line on the upstream side of said first valve as regards direction of flow of liquid in said pipe line with said inlet, a second conduit communicating said motor valve with said pipe line on the downstream side of said first valve, a third conduit communicating said check valve with said second conduit, a second valve in said second conduit intermediate the points of communication of said third conduit and said pipe line with said second conduit, and a differential pressure controller, said controller being operably connected with said second conduit on either side of said second valve and said controller being adapted to actuate said motor valve in response to pressure differential across said second valve.

8. In a pipe line pumping system, an apparatus :Eor reducing the power required by a multistage centrifugal pump operated at a constant speed by a constant speed prime mover when pumping less liquid than pump capacity at said constant speed, comprising, in combination, a pipe line, said pipe line being adapted for long distance transmission of liquids, a first valve in said pipe line, a multistage centrifugal pump, a constant speed prime mover, said prime mover being connected to said pump to rotate the impellers of said pump at a constant speed, an inlet in the first stage of said pump forliquid to be pumped, a first outlet in the last stage of said pump for outlet of pumped liquid, a second outlet in an intermediate'stage for outlet of pumped liquid, a check valve in said second outlet to permit flow of liquid only from said second outlet, a slow operating motor valve in said first outlet, a first conduit communicating said pipe line on the upstream side of said first valve as regards direction of flow of liquid in said pipe line with said inlet, a second conduit communicating said motor valve with said pipe line on the downstream side of said first valve, 21 third conduit communicating said check valve with said second conduit, a second valve in said second conduit intermediate the points of communication of said third conduit and said pipe line with said second conduit, and a. difierential pressure controller, said controller being operably connected with said second conduit on either side of said second valve, a reversing contactor operably connected with said controller and with said motor valve, said controller in response to a pressure drop across said second valve being adapted to actuate said contactor, and said contactor being adapted to open and to close said motor valve.

9. In the apparatus of claim 6 wherein said second valve is a manually operable valve.

10. In the apparatus of claim 6 wherein said second valve is a remotely operable motor valve.

References Cited in the file of this patent UNITED STATES PATENTS 1,231,613 James July 3, 1917 1,465,401 Losel Aug. 21, 1923 1,879,545 Seeley Sept. 27, 1932 2,660,121 Curtis et al. Nov. 24, 1953 2,819,836 Eberle Jan. 14, 1958 

